What! Quartz Tuning Forks?

No, I am not kidding!

How many of you knew that the resonator used in many quartz watches is actually a quartz Tuning Fork?

Accutron Watch Page reader and Physicist Erhard Schreck sent this excellent pic of a 32,768 Hz quartz crystal with its cover removed, to show a typical example of a quartz watch tuning fork. Next to it is a 1 cent coin to give you an idea of size.

qtf.jpg - 28.83 K

So why do we ignore quartz watches here, even though they are tuning fork watches? I guess the distinction we Accutron enthusiasts draw between quartz resonators and the metal resonators in Accutrons is in the way the energy of vibration of the fork is used, not only to keep the resonator vibrating, but to extract the energy to operate the watch. A very brief comparison follows:

The Quartz Tuning Fork Watch

In a quartz watch, the fork is kept vibrating by an oscillator circuit which supplies a voltage to a thin-film metal layer deposited on both sides of the crystal. This takes advantage of the piezo-electric nature of quartz to cause it to vibrate, and the quartz crystal itself is a capacitive component of that oscillator circuit. The varying voltage of this circuit is detected, and then divided down electronically to become a 1Hz signal which is used to drive a stepper motor.

A block diagram of a quartz watch may look like this:

Drive circuit -> Quartz Resonator -> Frequency divider -> Stepper motor -> Gear train ->Hands.

All of the interesting stuff happens in the electronics, which of course is invisible to the naked eye. The 1 Hz pulsed stepper motor and relatively low-ratio gear train driving the hands is completely boring and unexciting to us real watch enthusiasts, especially so because the stepper motor usually cannot be seen anyway. In fact, some quartz watches only pulse the stepper motor every 20 seconds. Nor is any intervention by the watchmaker possible here either, excepting for the replacement of complete modules or cleaning.

The Accutron Tuning Fork Watch

In an Accutron tuning fork watch, the fork is likewise kept vibrating by an oscillator circuit, though in this case the fork is an inductive component of that oscillator. But this is where quartz watches and Accutrons go their own sweet ways, because the energy of the Accutron fork is directly used to drive the gear train, extracted mechanically by the index mechanism. The fork and index mechanism combined can be thought of as a stepper motor. The electronics do nothing but keep the fork vibrating.

A block diagram of an Accutron watch may look like this:

Drive circuit -> Tuning Fork/Index Mechanism -> Gear train ->Hands.

All of the reduction occurs in the gear train driving the hands. A lovely balance of precision mechanical engineering and simple electronics!

Regardless of those technicalities....

the Bulova Accutron is still the world's first true electronic watch, and as such has a special place in technological history. The Accutron is the result of skilled craftsmanship, talented designers, watchmakers, engineers and draftsmen. No fancy computer-aided design went into these watches, and as a result they contain a little bit of the personality of all the men and women involved in their creation. Perhaps this is one of the reasons why quartz watches, which are mainly designed on computers, are considered by watch collectors and enthusiasts the world over as "soul-less".

Some Quartz Theory:

For those of you who wish to know how the dimensions of Quartz Tuning Forks are calculated, Erhard has kindly provided us with the basic formula:

Typical dimensions for watch tuning forks are as follows:

quadratic cross-section
l = length = 3mm
a = thickness = 0.3 mm
E = youngs modulus = 1*10^11 N/m^2
rho = density = 2500 kg/m^3

If we use the above numbers in the formula for a vibrating cantilever:

f=frequency = alpha/(2*pi) * a/length^2 *sqrt(E/(12*rho)) [Hz]
with alpha = 3.52 for the fundamental frequency, we get about f=34 kHz, which is close enough for me to the 2^15 = 32768.